Topics Covered in this Lecture:
Slideshow: Plate Tectonics
- The Major Ocean Basins and Their Features
- Oceanic Ridges and Submarine Trenches
- How Do We See the Bottom of the Sea?
Recent satellite studies and deep-sea mapping programs have revealed in dramatic fashion that he ocean floor is not flat. Indeed, the terrain of the ocean floor surpasses that of the continents, in terms of heights of mountains and depths of valleys. We will explore this terrain and equip ourselves with an arsenal of names and definitions that will help us find our way around this submarine landscape.
The Major Ocean Basins and Their Features
The Archaean Earth (3.5 - 2.5 billion years ago) was composed of a loose assemblage of continents surrounded by a vast ocean known as Panthallassa. As the supercontinent Pangaea drifted apart (most recently) about 280 million years ago, the beginnings of the Atlantic and Indian Oceans were formed in a body of water known as the Tethys Ocean. The split up of Pangaea into Gondwanaland and Laurasia, and eventually the six major continents as we know them, led to the formation of the three major oceans, the Pacific, the Atlantic, and the Indian.
The oceans (and adjacent seas) cover approximately 71% of the Earth's surface. Yet the distribution of this water across the planet is not equal; the Southern Hemisphere contains most of the water. The largest ocean on Earth, the Pacific Ocean, covers about 32% (one-third) of the Earth's surface. The Atlantic and Indian Oceans are about half the size of the Pacific Ocean, covering about 16% and 14%, respectively. Interestingly, the average depth of these oceans is about the same, roughly 12,400 feet deep.
There are also differences in the shapes of the basins and where they are located. The Pacific Ocean basin appears as one big round ocean, stretched across both hemispheres. Because of its shape, some scientists believe that the Pacific Ocean basin gave rise to the moon as the result of an impact with a large asteroid (or small planet!), which spilled the lighter contents of the Earth's molten crust into outer space. Future research in the deep basins of the Pacific or on the moon might shed some light on this mystery.
On the other side of the globe, the Atlantic Ocean basin looks pinched in the middle, where the western bulge of Africa reaches out towards the eastern bulge of South America. Then we have the Indian Ocean basins, which rests for the most part in the Southern Hemisphere, with only a very small portion extending above the equator. While the Atlantic Ocean basin and the Indian Ocean basin hold nearly the same volume of water, that of the Atlantic has been stretched across the globe while that of the Indian is confined to the southern half of the globe.
Despite these differences, ocean basins have many features in common that are useful to know. In general, ocean basins can be divided into two parts, the continental margins and the deep-ocean basins. The continental margins include the continental shelf and the continental slope. The deep-ocean basins comprise the "floor" of the sea, though it is hardly flat! As we shall see, the ocean floor consists of the oceanic ridges, submarine trenches, abyssal plains, guyots, island arcs, and a few other interesting geological features.
The continental shelf is the part of the continental margin nearest the shore, often flat and quite gentle in slope. These are actually parts of the continent and have been exposed and submerged throughout geological time depending on the sea level. This is where you stand when you are at the beach. The width of the continental shelf is highly variable, depending on whether it is on the leading edge of a continent, the edge, which moves towards an ocean basin, or the trailing edge, the edge that moves away from the ocean basin. On leading edges, continents are colliding and overriding the oceanic plates, leading to narrow continental shelves such as are found along the Pacific coast. Trailing edges are regions from which the continent moved away, leaving a wide swath of sediments behind it. This is typical of the continental shelf along the Atlantic coast.
The continental slope is the part of the margin where the slope drops sharply at a steep angle (about 4 degrees), in a region called the shelf break. The slope marks the region where the continental and oceanic crust meet. Continental slopes, being steep, give rise to avalanches of sediments called turbidity currents. When large accumulations of sediments are deposited on the shelf, they suddenly give way like an avalanche. These turbidity currents may cut into the sides of the slope, forming underwater canyons called submarine canyons (don't confuse these with submarine trenches, which are tectonic). One famous submarine canyon is the Monterey Submarine Canyon, which is as deep and steep as the Grand Canyon. Also, we should note that as these sediments descend to the ocean floor, they spread out forming alluvial fans and fan valleys, such as what you see at the mouths of rivers.
The continental rise is this is a region beyond the continental slope, which slowly descends into the oceanic basin. It is largely composed of sediments, which have flowed down the continental slope onto the oceanic basin. Because these sediments are often deposited by turbidity currents, they have been called turbidite deposits. One feature of turbidite deposits is that they are graded. That means that the heavier rocks and sands are found at the bottom and the finer sediments are on top. Where several turbidite deposits have been deposited, this pattern will repeat itself over and over again.
The ocean floor is the bottom of the ocean, beyond the continental margins. The ocean floor is not flat and the composition of rocks here is distinctly different than that of the continents. Though we may not realize it, the ocean floor covers more of the Earth's surface than the continents. In fact, in many places, the ocean floor has large expanses that are absolutely flat, flatter than anywhere on land. These regions are known as the abyssal plains.
The ocean floor is home to many other interesting geologic features. There are several regions along the ocean floor, which are, elevated a mile or so from the rest of the ocean floor. These are called oceanic plateaus. They are typically thicker and more "buoyant" than oceanic crustal material. It is thought that these may be fragments of continental crustal materials floating within the plates, or some may be the result of volcanic activity. Whatever their origin their distribution is widespread.
In some spots, ocean basins are covered with hills, called abyssal hills. Larger "hills", called seamounts, may also be present. These are formed by volcanic activity where "hot-spots" occur. While the source of these hot spots is not known, there appear to be particular locations anchored in the mantle which push magma through the oceanic crust towards the surface. These hot spots give rise to undersea volcanoes, which sometimes rise to the surface and form islands.
Another feature attributed to hot spots are island arcs. These arcs appear as a series of volcanic islands in close association with each other. The Hawaiian Islands are a good example.
If the volcano remains submerged, they become known as seamounts. Seamounts are present in many of the ocean basins. If the volcanoes break the surface of the ocean, they become islands. The weight of these volcanoes on the crust causes the plate to sink. Oftentimes, the tops of these volcanoes are weathered by wind and waves, and the tops become flat. When these flat-topped volcanoes submerge completely, they become known as guyots ("ghee-oats"). Eventually, the Hawaiian Islands, which are slowly sinking, will be completely submerged and turn into guyots.
Geological activity is not the only type of activity that creates structures on the ocean floor. Biological activity may contribute as well. Among the best known are the coral reefs, such as the Great Barrier Reef in Australia.
The most famous "marine carpenters" are the corals. Corals, which are related to sea anemones, remove calcium from the seawater to form intricate and often massive calcified skeletons. Corals consist of colonies of individuals, known as polyps, each of which contains a number of symbiotic microscopic plant cells, called zooxanthellae. The zooxanthellae, which are photosynthetic, provide some of the energy and biochemical building blocks, which help the polyps, build their coral skeletons.
Some of the best-known structures built by corals are the fringing reefs. Fringing reefs are most typical around islands, but also may be formed along coastlines. They usually occur where the depth of the seawater is great enough that they don't dry out and shallow enough that light penetrates to their zooxanthellae. Oftentimes, the actions of these organisms may be substantial enough to build barrier reefs, which form a barrier between the shoreline and the ocean. The most famous of these is the Great Barrier Reef in Australia.
Another curiosity which can be attributed to corals are atolls. Atolls are small ring-shaped reefs, which occur in the middle of the ocean. The processes, which form atolls, were discovered by Darwin, who correctly postulated and later confirmed that atolls were formed as volcanoes sank back into the sea. The reefs, which formed around the ring of the volcano, were able to grow as fast as the volcano sank thus creating a ring of coral, which stayed at the surface of the ocean. Because conditions at the other edges of the atoll, i.e. the side facing the ocean, are better for coral growth than along the inner edge, the ring also grows outward, thus forming the familiar ring-shape structure of the atoll.
Question: How are ocean basins like a bowl? How are they different?
Oceanic Ridges and Submarine Trenches
The most notable and significant structures known to our planet are the oceanic ridges and submarine trenches, the mountains and valleys of the ocean floor.
A careful look at a bathymetric map of the world ocean will give you some insight into the importance of these structures. Starting in the North Atlantic, note the "mountain range" or ridge that bisects the basin from north to south, turns the corner around the Cape of Good Hope in Africa and ends up in the middle of the Indian Ocean. Starting there, with the ridge that bisects the Indian Ocean, trace your finger south and east towards Australia, circling the bottom of the Pacific basin and charting a staggered northeasterly course bisecting the Pacific on a line that "ends" in Baja California.
These mid-oceanic ridges have been called "the largest mountain chain and the most active system of volcanoes in the solar system." These ridges are formed as molten rock rises from the Earth's mantle and fills in where boundaries occur between continental plates. The height of these ridges depends on the rate at which magma is forced upwards. A very high rate of magma supply results in a ridge that pushes above the surface of the sea, creating islands like Surtsey near Iceland. In fact, Iceland itself is part of the Atlantic mid-oceanic ridge and is one place in the world where you can observe a mid-oceanic ridge on land!
The oceanic ridge system wraps around the globe like the seams of a baseball and has been called the "zipper" of the Earth. The entire system is 43,470 miles long but only 3-19 miles wide, much thinner than the width of the plates. In some places the ridge is discontinuous and broken into sections. These sections are visible as lines that cross perpendicular to the main ridge, and are known as transform faults. One other feature of oceanic ridges is the presence of "valleys" along the top of the ridge, a depression runs along the top of the mountain ridge. This is known as a rift valley.
One other important feature of oceanic ridges is the presence of hydrothermal vents. These are places where seawater seeps down into the rocks where it becomes heated and is forced out like steam. These hydrothermal vents have been nicknamed such things as "black-smokers". In recent years, scientists studying the ocean bottom discovered entire communities of organisms associated with these vents, called vent communities. In some cases giant worms were found over ten feet long. These communities continue to be a subject of active scientific research and we will learn more about them at the end of this segment on geophysical processes.
As a counterpart to oceanic ridges, we also have submarine trenches. Submarine trenches can be found off the continental shelf of Alaska, heading west towards Japan, then tracing a deep rut into the southwestern Pacific towards New Guinea and the Philippines. Other trenches are visible off the coast of New Zealand, Central and South America, and in the Mediterranean Sea.
Among the best known of all submarine trenches is the Marianas trench, located in the Western Pacific Ocean east of the Philippines. This trench is approximately 1,600 miles long and contains the deepest known spot in the ocean, known as the Challenger Deep. Occupying a portion of the Marianas Trench, the Challenger Deep was most recently measured with a Japanese Remotely Operating Vehicle (ROV), called Kaiku. When Kaiku landed on the bottom of the Challenger Deep, its depth sensor read 10,911 meters (35,788 feet), or 6.78 miles. Photographic observations in the near vicinity revealed a hovering lugworm and several abyssal ghost shrimp. This was very near to the deepest point, which is reported at 11,033 meters (36, 198 feet), or 6.86 miles.
While we're comparing sizes, we should make mention of Mauna Loa in Hawaii, which is the tallest mountain on Earth (though not the highest). From its base deep in the ocean (16,400 feet) to its often snow-covered top (13,677 feet), this mountain in the sea measures out at 30,077 feet, or 5.7 miles high. Compare this with Mount Everest in Nepal rises to 29,028 feet, or 5.49 miles above sea level.
Where do these mountains and trenches come from? The answer lies in the theory of plate tectonics, which says that the earth's crust can be divided into a series of discrete "plates" that float on the earth's molten mantle. The forces, which drive these plates, such as flows of magma upward in the region of oceanic ridges, cause them to bend and buckle and jostle against each other. Where plates spread apart from each other, oceanic ridges arise. Where they are thrust beneath each other, submarine canyons are formed.
Question: How different would the Earth look without the water? Do mountains erode more slowly or more quickly beneath the sea? Why?
How Do We See the Bottom of the Sea?
In the past decade, our ability to map the bottom of the sea has become quite sophisticated. Let's take a few minutes to acquaint ourselves with the latest technological advances for mapping the ocean floor.
All of us are probably familiar with sonar, which stands for Sound Navigation Ranging. Sonar has been around since the 1940s, but since many of the new methods are based on sonar, it is useful to briefly review the principles here. Simple sonar consists of a sound source, the familiar "ping" of all those submarine movies, and a sound receiver. By knowing the speed of sound through water (which is about 4500 feet per second), and the length of time between the "ping" transmission and the "ping" return, the depth of the bottom can be estimated. One of the major limitations of this method is that it is effective only to depths of about 6000 feet, considerably less than much of the ocean bottom.
Two new techniques for mapping the sea floor have recently been developed. These are side-scan acoustical imaging and swath beam mapping. Both of these methods employ multiple "pingers", and rely on sophisticated electronics and 3-D computer algorithms for producing very clear 3-dimensional images.
Side-scan measurements typically use a towed instrument that sends out multiple narrow beams of sound, called footprints, to scan features of the ocean floor. This type of measurement is used most often to find shipwrecks, downed planes, or other structures at the bottom of the sea.
Another recent innovation for mapping the sea floor is called multibeam echosounding or swath beam mapping. In this technique, multiple simultaneous beams of sound are directed to the bottom which are received on their return by a sophisticated multidirectional receiver. By using computers which are capable of handling large streams of data and creating complex computer programs for unraveling these signals, ocean scientists have been able to produce 3-dimensional images of the ocean floor much faster and with much higher resolution than ever before.
Finally, while it may not seem so obvious at first, satellites are now being used to map the bottom (yes, the bottom) of the sea. Because water is virtually incompressible, ridges and trenches on the ocean floor are reflected as small bumps and depressions on the ocean's surface. While the resolution is limited, very good images of the ocean floor have now been made possible by this technique. Obviously, the primary advantage is coverage. A satellite can simply see much more of the world that a ship. The Topex/Poseidon satellite, launched a couple years ago and designed to measure centimeter-range changes in sea surface height, has already provided a new look at the ocean floor.
These new mapping techniques have once again opened a new world beneath the sea. Geologists are discovering a variety of new phenomena and seeing features of the ocean floor as easily as if they were flying an airplane over a mountain range. These new techniques promise to answer many questions about the structure and shaping of the ocean floor. Of course, as always, they are certain to raise new questions as well. New vistas await us in the landscape of the ocean floor!
Question: How does technology help us understand the oceans? Why is a 3-D picture better than a 2-D picture?
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